Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimination of porosity in castings, such as for instance turbine blades, in order to substantially increase their service life and strength, in particular the fatigue strength. Another field of application is the manufacture of products, which are required to be fully dense and to have pore-free surfaces, by means of compressing powder.
In hot isostatic pressing, an article to be subjected to treatment by pressing is positioned in a load compartment of an insulated pressure vessel. A cycle, or treatment cycle, comprises the steps of: loading, treatment and unloading of articles, and the overall duration of the cycle is herein referred to as the cycle time. The treatment may, in turn, be divided into several portions, or phases, such as a pressing phase, a heating phase, and a cooling phase.
After loading, the vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure and temperature of the pressure medium is then increased, such that the article is subjected to an increased pressure and an increased temperature during a selected period of time. The temperature increase of the pressure medium, and thereby of the articles, is provided by means of a heating element or furnace arranged in a furnace chamber of the pressure vessel. The pressures, temperatures and treatment times are of course dependent on many factors, such as the material properties of the treated article, the field of application, and required quality of the treated article. The pressures and temperatures in hot isostatic pressing may typically range from 200 to 5000 bars, and preferably from 800 to 2000 bars and from 300° C. to 3000° C., and preferably from 800° C. to 2000° C. respectively.
When the pressing of the articles is finished, the articles often need to be cooled before being removed, or unloaded, from the pressure vessel. In many kinds of metallurgical treatment, the cooling rate will affect the metallurgical properties. For example, thermal stress (or temperature stress) and grain growth should be minimized in order to obtain a high quality material. Thus, it is desired to cool the material homogeneously and, if possible, to control the cooling rate. However, it is also of importance not to increase the total manufacturing costs of a pressing arrangement and/or the costs associated with operating the pressing arrangement in too large extent in attempt to satisfy the requirements with regard to desired cooling rate and homogenous cooling.
Prior art hot isostatic pressing arrangements are often manufactured with uniform cylinder vessel walls and an outer cooling circuit in which a cooling liquid is circulated. Thereby, a transmission of heat or thermal energy through the vessel walls can be achieved. A traditional prior art pressure vessel cylinder is shown in FIG. 1a. The pressure vessel cylinder 1 is closed at the respective ends by means of upper and lower lids 2 and 3, respectively. Radial pre-stressing means 4a are provided around the envelope surface of the pressure vessel cylinder for accommodate radial forces exerted on the pressure vessel walls and axial pre-stressing means 4b are provided for accommodating axial forces exerted on the lids 2, 3. The radial pre-stressing means can be provided around the entire envelope surface of the pressure vessel cylinder. Due to the pre-stressing means 4a, 4b, the lids 2, 3 are capable of closing the pressure vessel 1 without any threading means or similar to attach the lids. Moreover, the outer wall of the pressure vessel 1 is provided with channels, or tubes, 5 in which a coolant for cooling may be provided. The coolant is preferably water, but other coolants are also contemplated. The flow of coolant is indicated in FIG. 1 by the arrows in the channels 5. During cooling, thermal energy is transferred from the warm pressure medium through the pressure vessel wall to the circulating cooling liquid. Furthermore, in order to be used in a pressing arrangement, the pressure vessel 1 is normally provided with means such a furnace, load compartment, heat isolation means etc., which not are shown in FIG. 1a for purposes of clarity.
In FIG. 1b, another prior art pressure vessel is shown. The pressure vessel 10 has a so called “dog-bone” design. This pressure vessel 10 is not provided with any pre-stressing means in this solution. In the illustrated configuration, the lids 12, 13 are attached to the pressure vessel 10 by means of threaded sections 14a and corresponding threaded sections 14b of the pressure vessel 10. Because there is no pre-stressing means for accommodating radial and axial forces exerted on the pressure vessel 10 and on the lids 12, 13, the pressure vessel 10 has to be made stronger, in particular, at the end portions where the lids are attached. To absorb the significant axial load exerted primarily from the lids, the pressure vessel 10 is provided with thick walls at the portions at the upper and lower lid. Thereby, the pre-stressing means can be omitted in this design. As can be seen in FIG. 1b, the upper and lower end portions 16, 17, respectively, of the pressure vessel wall are significantly thicker than the central portion 18 of the pressure vessel 10, which has a reduced thickness to save weight. A relation between outer diameter, od, and inner diameter, id, (od/id) is at least 1.2 (and often up to 1.3-1.4) at the central portion 18 where the vessel 10 has its thinnest wall thickness. At the thicker portion of the pressure vessel wall 16, the relation between outer diameter, od, and inner diameter, id, (od/id) is about 1.4-1.9. The significant radial and axial forces that have to be absorbed by the pressure vessel 10 require such high diameter relation od/id.
To provide an enhanced cooling capability, cooling elements are arranged in connection to the outer wall of the pressure vessel 10 in which a coolant is circulated. The coolant is preferably water, but other coolants are also contemplated. During cooling, thermal energy is transferred from the warm pressure medium through the pressure vessel wall to the circulating cooling liquid.
However, these prior art pressure vessels are impaired with drawbacks. The traditional uniform pressure vessel provided with axial and radial pre-stressing means may not provide a sufficiently rapid cooling without additional means for achieving such enhanced cooling. For example, heat exchangers have been suggested for that purpose. A heat exchanger arranged inside the pressure vessel do on the other hand add complexity in that, for example, pipes for supplying cooling medium has to be arranged in though holes of the pressure vessel. This may also entail increased maintenance needs.
The “dog-bone” solution, on the other hand, is very heavy due to the wall thickness despite the reduced wall thickness at the central portion.
To conclude, there is therefore a need within the art of improved pressure vessels for pressing arrangements capable of controlled, rapid and homogenous and cooling of articles and pressure medium.